Humidity sensors are widely utilized for humidity and air quality measurement in a variety of applications (e.g., automotive for comfort, safety and power train, home appliances for moisture and temperature control, energy efficiency, humidity switches, HVAC, etc.). Humidity can be measured utilizing a number of techniques such as, for example, wet bulb psychrometer, chilled mirror dew point, saturated salt solutions, resistive RH sensors, capacitive RH sensors, and SAW-based humidity.
The majority of prior art capacitive humidity sensors with on-chip signal conditioning internally connected with a humidity capacitive element possess advantages such as a high signal-to-noise rate, low cost, and small size due to the elimination of wire connections between the capacitive element and separate supporting circuitry. Such capacitive humidity sensors, however, possess a number of disadvantages such as a slow response time, inherent accuracy issues (e.g., 2% or less), device degradation over a long period (e.g., over a 5 year period) and a relatively high hysteresis. These disadvantages can constrain the application of the capacitive humidity sensor in high-end applications such as, for example, weather stations, instrumentation and industrial control in semiconductor foundries.
One solution to this problem involves utilizing a SAW-based humidity sensor with approximately a 1 second response time and 1% accuracy for high-end applications. A SAW sensor, however, utilizes a bulk piezoelectric material (e.g., LiNbO3, LiTaO3, quartz etc.) as a substrate to transform the surface acoustic wave. The piezoelectric materials are not semiconductor circuitry compatible materials, although they are good acoustic transmission materials.
Hence, it is not possible to integrate the signal conditioning circuitry on the same chip, as SAW sensing devices rely on piezoelectric materials to transform the electrical energy to surface acoustic waves and to interrogate with the humidity sensitive film and also sense the humidity value tested. Therefore, separate signal conditioning circuitry must be developed. Additionally, the SAW sensing element and signal conditioning circuitry must be packaged and bonded together to generate a meaningful output, which is proportional to the humidity value tested. Such a solution possesses inherent disadvantages such as, for example, a low signal-to-noise and is relatively complex and expensive to package.
SAW-based sensors to date also include one set of interdigital (IDT) transducers and a humidity sensitive film deposited on the top of a surface acoustic wave media. When the humidity sensitive film absorbs the moisture, the mass will change, thereby altering the frequency at the receiving IDT. However, because the output frequency of the IDT is also sensitive to temperature and pressure, a single IDT set cannot distinguish between differences in pressure, temperature and humidity values.
Based on the foregoing, it is believed that a need exists for an improved SAW-based humidity sensor with integrated signal conditioning that is capable of being configured and/or located on the same substrate. A need also exits for designing a common mode delay line in order to eliminate common mode noise, as described in greater detail herein.